An escape sequences is a series of non-printing characters, beginning with an Escape character, that sends commands to devices. Escape sequences are used for printing, communications, and display management. The command in an escape sequence results in specified actions by devices. Escape sequences are also called control codes or control sequences.

Escape sequences are most often used in scripts, as in the following examples:

cursoroff: echo "ESC[?25lcursor is off"

cursoron: echo "ESC[?25hcursor is on"

The NCD Terminal Emulator includes a subset of the VT320 command set, as well as NCD-specific sequences. These escape sequences, their actions, and associated functions are listed in Table 12-5. Some of the sequences are described in more detail following the table.

The table lists sequences that differ depending on whether the environment requires eight-bit or seven-bit mode. The eight-bit mode sequence (for example, CSI. . .) is listed first, followed by the equivalent seven-bit mode sequence (for example, Esc[. . .).

Numerical variables are represented as pn. Variables representing a number of rows or columns are represented as pr or pc, respectively. Variables requiring a parameter setting from a number of specific choices are represented by ps. Other variable types are defined in the table as required.

Table 12-5 Escape Sequences

The status line is referred to in Digital documents as "the 25th line of the display." Because NCD Terminal Emulator windows may have more than 24 lines, the status line must be treated as the hardware status line instead of line 25.

The NCD implementation of the status line allows the programmer to create a new line that appears at the bottom of the window. Normal programmatic editing operations are available for this line, but different character modes (such as blinking or bold) are not supported. To enter characters into the status line, the programmer switches from the main display to the status line display, then uses normal cursor control and text to add characters.

By default, the status line is not visible on the screen.

To use the status line, enable or disable it with the following sequence:

CSI ps $ -

where ps indicates which status line to use (or none):

0	No status line available
1	Indicator status line (no-op)
2	Host-writable status line

Hence, the value 2 makes the status line appear, and 0 makes it disappear. Note that content is not retained when the status line is hidden; the line is emptied.

To select the status line, use the following sequence:

CSI ps $ }

where ps represents the display area to which the terminal sends data:

0	Send characters to the main display
1	Send characters to the status line

Once the status line is selected, all character input is directed there until the main display is selected.

Programming Function Keys on N-108LK Keyboards

Fifteen of the twenty function keys on the N-108LK keyboards can be redefined by the user. The definable function keys are:

• F6 through F14
  

• Do and Help
  

• F17 through F20
  

When redefined, the shifted state of these keys takes on the defined values. The unshifted keys still work as usual; you cannot programmatically rebind the unshifted state.

The two permissible formats of the escape sequences follow:

DCS  pc ; pl | Ky1 / St1  ; ...  Kyn / Stn  ST
DCS  pc ; pl | Ky1 \ St1 ; ...  Kyn / Stn  ST

The parts of a function key definition are:

DCS	Device control string
pc	Clear parameter:
	0	Clear all keys before starting (reset)
	1	Clear one key at a time, as overwritten
pl	Lock parameter (no-op on NCD terminals):
0	0	Lock the keys
	1	Unlock the keys
Ky1/St1 or Ky1\St1		Key definition string. There can be n of these, separated by semicolons. The format is a key selector number, a slash, then the rebinding. (See Table 12-6). The Ky1/St1 version requires that you supply the hexadecimal values of the letters in the key definition string. NCD has added another option, Ky1\St1, which allows you to supply ASCII characters for the string.
ST		String terminator character, or ESC \

Table 12-6 Key Selector Numbers

Examples of function key definitions follow (spaces have been inserted for legibility, but must not be included in the definition):

DCS 0 ; 1 | ST	Clears all key rebindings
DCS 1 ; 0 | ESC \	Locks keys (no-op on NCD terminals)
DCS 1 ; 1 | 34\ Print ST	Rebinds F20 to string "Print"
DCS 1 ; 1 | 34/5052494E54 ST	Rebinds F20 to string "Print"

In the Digital implementation, there is a limitation of 256 characters combined for all programmable function keys. NCD has a limitation of 256 characters per rebound key.

With Digital computers, key locking and unlocking can be set through hardware. Because NCD does not have this hardware, this function does not work. Locking is not enforced.

Configuring Window and Icon Titles

Icon and window titles can be configured through the following escape sequence:

OSC ps ND string NP

where:

ps	Determines how the string is used:
	0-Uses string as the new icon name and window title
	1-Uses string as the new icon name only
	2-Uses string as the new window name only
ND	Is any non-alphanumeric character (and is discarded)
string	Becomes the icon name and window title (or icon name only). This is an ASCII printable string that contains a maximum of 511 characters.
NP	Is any non-printing character (and is also discarded)

Configuring Color Text

You can specify color text programmatically. This feature is based on the SGR (select graphic rendition) paradigm used to control blinking, bold, inverse, and other text attributes.

There are two methods of setting text color:

• Using a subset of the ISO 6429 standard for selection of basic colors for foreground and background
  

• The Hewlett-Packard color-pair model in which you can select pairs of any X colors for text (X colors are listed in the file /usr/lib/X11/ncd/rgb.txt.)
  

The color selection code is limited to eight combinations of colors at any given time.

The number of combinations can be effectively doubled by using the inverse graphics rendition, but the number of colors that can be displayed on the screen simultaneously is limited.

The ISO specification defines SGR sequences to change the foreground and the background pens, as listed in Table 12-7.

Table 12-7 SGR Sequences for Foreground and Background Pens

For example:

CSI 31 m or ESC[ 31 m

renders foreground text in red, and

CSI 44 m or ESC[ 31 m

renders background text in blue.

These selections can be mixed to use combinations of the foreground and background colors when rendering text. However, only eight combinations are permitted simultaneously on the screen. The eight color cells are reused as needed.

Note also that the combination of black text on a white background is always reserved for the first color cell; hence, there are really only seven combinations of these colors that you can select.

If you attempt to use more than seven combinations at one time, the resulting text is displayed using color cell zero (black on white).

The eight available color cells can be assigned to any combination of foreground and background colors using NCDware-specific functionality.

This functionality builds on the SGR method of text specification with the addition of the range of new selections listed in Table 12-8

Table 12-8 New Color-Pair Selection

For example:

CSI 91 m or ESC[ 91 m

uses color-pair1 for rendering text.

Specify the colors associated with the selections by using an extension of the OSC functions in the following format:

OSC ps ND string NP

where:

ps	ranges from 90 to 97 for the color-pair selection
ND	is any non-alphanumeric character (and is discarded)
string	is in the format foreground/background
NP	is any non-printing character (and is discarded)

For example:

OSC 91 ; orange/brown ^G

or

ESC] 91 ; orange/brown ^G

sets color-pair1 to foreground orange, background brown.

The first color, color cell 0 (zero), is special-it is the default used for normal text. In addition, the background of the window is reset to the background specified with this color. In the ISO 6429 model, color cell 0 is always used for text specified as black text on a white background. Note that this may have been changed using the Hewlett-Packard method, which results in black text on a white background being stored in color-pair0.

You can use the two models together, but this is somewhat tricky in terms of cell reuse. The zero cell is never reused, but others are available to be reused if a free cell is needed and there are no matching colors for an existing cell on the screen. (The code scans the screen to determine if a color cell is in use when it needs to allocate a new color selection.) As a rule, you should limit color to eight combinations at any given time, or twice that using inverse text. If you attempt to use more colors, the result is plain color text.

Set up new color cells as early as possible and not within loops.

Reverse video inverts all the colors that are set programmatically. The user can select reverse video from the Options menu or use the reverseVideo resource.